need help deciding what pitch prop to get??

[matt88]

Motor is a 1973 85hp merc. Prop on it now is a 21pitch but is not in good shape at all. Merc is hung on a 15ft ski boat with a dry weight of 750lbs and gross weight of 1400lbs. Top speed with this prop is 35mph at 5600 rpms. The motor height needs to be readjusted again as well. I can trim it out to the hole before the last one but there is a lot of spray coming from the cav plate so I guess I need to raise the engine a little more. When I got this data the boat was fully loaded with fuel,gear,etc... The motor may actually turn a few more rpms because after my helper bailed on me I discovered I had the engine running a little lean and it seemed to pick up a little power after I tuned the carbs on the water instead of the driveway. Need to test that again I know but I just wanted a second opinion before I buy a prop. I know I need to straighten out a few more issues like the motor height,etc... but I have to get this damaged prop off the boat to get some accurate numbers so is a 23 pitch enough or should I go for the 25?? My goal here is to achieve as much top end as possible with a full load of fuel,fishing gear+2 people,me and 1 passenger.

 

[Bifflefan]

Re: need help deciding what pitch prop to get??

1400 lbs is not much with an 85 HP.
Something is not right with your numbers.
I think the 21 should do fine if it was any good. My old 16' with an 85 HP used to run 40+ with a 17p at about the same to less RPM if memory serves me. Mine had plenty of power and speed. I never tried a 21p but had a 19p that I used a couple times.
Is there a prop shop in your area that will let you rent, or try before you buy?
They can usually fix your old prop for way less than a new one.
Got the motor height right before you start with the prop.

 

[matt88]

Re: need help deciding what pitch prop to get??

The numbers on the data plate read
85
750
1240 (instead of 1400)
Im assuming that this means 85 max hp
750 net hull weight
1240 gross.

I have talked with two people one that had a boat like mine (with a 75hp)and one that has a 17ft hydrostream with an 85hp merc. Both have told me that my boat should do 55-60mph. Im attempting to be conservative with the 23p because with my 2.3 lu gear ratio that would only put it in the low 50mph range. I would like to see higher speeds but I need a prop that I know I can use now and try to fine tune later. Your 85hp probably had a 1.78 gear.